A phased array antenna is capable of forming a beam in different directions without moving the antenna mechanically. The beam is electronically steered or moved by changing the relative phase between each element in a prescribed manner, through a device called a phase shifter or phasor. The phase shifter is usually composed of three sections as shown typically in FIG. 1. As shown in FIG. 1, a microwave section 10 is, typically, a waveguide housing that is ferrite toroid controlled or a stripline or microstrip that is diode controlled. This section changes the radio frequency phase through interaction with the electromagnetic fields. The driver 12 provides the electronic signals necessary to regulate the phase shift within microwave section 10. Driver 12 is shown as a Darlington transistor circuit and is typical of the high current circuit used to provide drive current to a ferrite toroid. The logic section 14 functions as the interface between the phased array controller and each individual phase shifter. In the logic section 14 the computer logic level commands from the controller are changed into trimming signals for the driver to use.
In a phase shifter the material characteristics, part types, and dimensions all play an important role in controlling of phase shift. Since phase accuracy directly affects not only beam pointing accuracy, but also the antenna gain and sidelobe levels, it is very important that phase shifters be controllable, reproducible devices.
The antenna designer is permitted two options in the control of the phase shift. The first option is to control all the parameters of the microwave section very carefully and thus produce very uniform parts. The other alternative is to build a less controlled part, measure its input/output characteristics and compensate for any errors. The compensation or trimming can be performed in either the microwave section 10 or in the driver section 12. Microwave trimming can be performed by varying the amount of phase shifting material used or by introducing new material into the device. Driver or electronic trimming takes the phase shift command and modifies it to account for the variability in the microwave section. One method being used for electronic trimming is to store a digital word in the beam steering computer memory and adjust the commands before they are sent to the phase shifter. Another electronic trimming technique involves storing a digital word on board the driver and adding this word to the command within the driver logic. These techniques have been used with digital control lines to the phase shifter and require a command line to each individual phasor.
An alternative method to command an array is to use row and column steering. Rather than having an individual command line to each phase shifter, row and column steering has a single command line for each row and each column of the array lattice. Such a scheme provides economics in that the number of command lines is reduced from N.sup.2 to 2 N for a square array with N elements on a side. Since the phase shift requirements to steer beams are linear, row and column steering performs the commanding function ideally. In addition, a technique has been developed whereby the phase shift command is the combination of a start pulse sent on either the row or column line, which turns the driver on, followed at an appropriate time by a stop command on the other axis line, whereby the driver is turned off. This method is shown typically in FIG. 2 for a 9 element array (3.times.3). Each element receives a command for the row line and the column line. The actual phase command to each element is the sum of the phase command on the row line plus the phase command on the column line. As an example, assuming, the column line starts or turns on the driver and the row line turns off or stops the driver current, a pulse on C1 would start current flowing into the phasor of element 1 and a pulse on R1 would stop the current flow. These pulse commands are shown in FIG. 3(a). FIG. 3(b) shows a typical command given to element 2 during the same cycle. The command occurs at the same time at both elements 1 and 2; however, element 2 is started by a different column command (C2) that occurs later than the column command to element 1 (C1). Thus the current to element 2 is on for a shorter time than the current to element 1. Since the amount of phase shift (.phi.) is directly proportional to current (I) and time (t), EQU .phi..alpha.It, (1)
the phase of element 1 is longer than that of element 2.
The phase settings at each phase shifter need only be between 0.degree. and 360.degree. because the sinusoidal wave transmitted or received by the array repeats every 360 electrical degrees. Consequently it is only necessary to build a device that is capable of shifting 360.degree. in the maximum time between stop and start commands.
Additional prior art background respecting phased array antennas and beam steering control is set forth in technical publications such as the "Radar Handbook" by M. I. Skolnik, published by McGraw-Hill, Book Co., 1970, Chapter 12. Pages 193-197 of the "Handbook of Radar Measurement", by D. K. Barton and H. R. Ward, Prentice-Hall, Inc., Englewood Cliffs, N. V., 1969, are also descriptive.